Un null_vptocnp(), cache vp->v_mount and use it for null_nodeget() call.

The vp vnode is unlocked during the execution of the VOP method and
can be reclaimed, zeroing vp->v_data. Caching allows to use the
correct mount point.

Reported and tested by: pho
PR: 235549
Sponsored by: The FreeBSD Foundation
MFC after: 1 week